1. Field of the Invention
The invention relates to a method of manufacturing steel sheets by a rolling process so that the steel sheet so produced can be subjected to a forming process in the as-rolled condition.
2. Related Art Statement
Steel sheets of this general type having generally a relative thin thickness of not more than 2 mm, which are used in building materials, automobile components, various surface treating black plates and the like, are required to have the following properties:
(1) Mechanical Properties
In order to obtain good bending formability bulging formability and drawing formability, the steel sheet is mainly required to have high ductility and high Lankford value (r-value). In this case, r-value is represented by r=(r.sub.L +r.sub.C +2r.sub.D)/4, wherein r.sub.L, r.sub.C and r.sub.D are r-values in a rolling direction (hereinafter abbreviated as L-direction), a direction perpendicular to L-direction (hereinafter abbreviated as C-direction) and a direction inclined at 45.degree. with respect to L-direction (hereinafter abbreviated as D-direction), respectively.
In order to increase the yield of steel sheet during the forming process, the process known as bulging is often adopted because the flow of material from the blank holding portion can be reduced using the bulging forming process. In this case, it is required to have a high n-value (strain hardening exponent) as a property of the material.
Even if the formability in a particular direction is good, the actual forming is plane, so that when the planar anisotropy is large, folds are produced after the forming. On the other hand, when the anisotropy is small, the amount of earing cut after the forming becomes less to reduce the blank area, so that the yield of steel sheet is largely improved. Such an anisotropy as a mechanical property can be evaluated by .DELTA.El (anisotropic parameter of elongation) and .DELTA.r (anisotropic parameter of r-value). Particularly, .DELTA.El.ltoreq.5% and .DELTA.r.ltoreq.0.5 are required as a steel having an improved anisotropy.
In the steel sheet of this type, a good balance of tensile strength and elongation is required because when the balance of tensile strength and elongation in poor, problems such as flange cracking and the like can be encountered during the forming process. A standard for providing a good balance of tensile strength (TS) and elongation (El) is approximately TS(kg/mm.sup.2).times.El(%).gtoreq.1,500.
When the formable steel sheet is held at room temperature for a long period of time, the age deterioration may be caused to bring about the degradation of formability and hence cracking may be produced in the press forming. For this reason, the aging resistance is important, whose standard is AI (aging index).ltoreq.4(kg/mm.sup.2).
In the steel sheet for automobile applications, the thickness of the sheet has been required to be reduced to improve fuel consumption of the vehicle. During the thinning of the sheet, a problem of reduction of tensile rigidity of the formed product is caused. For instance, when a force is applied externally to the formed product, deflection of the sheet is readily caused. Since the tensile rigidity of the steel sheet is proportional to Young's modulus, it can be enhanced by increasing the Young's modulus in the sheet plane. In this connection, the tensile rigidity is good when an average value (E) of Young's moduli in L-direction, C-direction and D-direction is not less than 22,000 kg/mm.sup.2. In this case, E is represented by E=(E.sub.L +E.sub.C +2E.sub.D)/4.
The automotive parts such as panel, oil pan, gasoline tank and the like are required to be severe in the formabilities, particularly deep drawability. For this end, the steel sheet used for such parts is required to have r-value of not less than 1.7 though it is dependent upon the form of the respective part.
On the other hand, the steel sheet for use in outer panels of the automobile is required to have a low yield ratio (YR, %) represented by an equation of YR=(tensile strength/yield strength).times.100, because when YR is low, it is possible to control planar strain in relatively light worked portions, for example, portion of a door outer near a handle. Further, there is a recent trend of enlarging the size of the panel for reducing the number of spot weld points and the like, and in this case the low YR is very effective for the press forming having a small planar strain.
(2) Surface Properties
Since the formable steel sheets are mainly used in outermost portions of final products, various surface treating properties are important in addition to the shape and surface appearance of the steel sheet.
Particularly, in the steel sheets for automobiles, the treatment prior to painting, phosphate coating is significant, becuase if the phosphate coating property is bad, sufficient baked-on painting property can not be ensured.
Further, the demand for the corrosion resistance of the formable thin steel sheet becomes more severe, while the use of surface treated steel sheet rapidly increases. Especially, the steel sheets for automobiles used in North Europe and North America should be durable to the corrosion due to the salt used for snow melting, which requires the more severe corrosion resistance. On the other hand, even when using the surface treated steel, if it is apt to be damaged in the forming, the corrosion resistance is deteriorated, so that the adhesion property between the base plate and the surface treated layer becomes very important in the surface treated steel sheet. Furthermore, since the formable steel sheet is used in the outermost portion of the final product as previously mentioned, the corrosion resistance of the steel sheet itself, particularly pitting resistance is important.
In general, the manufacture of such thin steel sheets is as follows:
At first, a low carbon steel is mainly used as a steel material, which is made into a slab sheet having a thickness of about 200 mm through ingot-making and slabbing. Then, the slab sheet is subjected to heating and soaking in a heating furnace and roughly hot rolled into a sheet bar having a thickness of about 30 mm. Next, the sheet bar is subjected to a final hot rolling at a temperature of higher than Ar.sub.3 transformation point to form a hot rolled steel sheet with a given thickness, which is then pickled, cold rolled to form a cold rolled steel sheet with a given thickness (not more than 2.0 mm) and further subjected to recrystallization annealing to obtain a final product.
A great drawback of this customary process is very long in the steps required to produce the final product. As a result, energy, labor and time required for the manufacture of the final product are vast, and also various troubles on the quality, particularly surface properties of the product are unfavorably caused through the long steps. For instance, there are unavoidable troubles such as occurrence of surface defects at the cold rolling step, concentration of impurity elements into sheet surface at the recrystallization annealing step, deterioration of appearance resulting from surface oxidation, degradation of surface treating property and so on.
As a method of manufacturing a formable thin steel sheet, it is also considered to provide a final product through only the hot rolling step. In such a method, the cold rolling step and recrystallization annealing step can be omitted, so that the industrial merits are large.
However, the mechanical properties of the thin steel sheet obtained only through the hot rolling step are fairly poor as compared with those obtained through the cold rolling-annealing steps. Although the press formable sheet used in the automotive vehicle body or the like is particularly required to have an excellent deep drawability, r-value of the hot rolled steel sheet is as low as about 1.0 and consequently the application of the latter sheet is considerably restricted. Because, in the conventional hot rolling method, the final temperature is higher than Ar.sub.3 transformation point so that the texture is randomized in the .gamma..fwdarw..alpha. transformation. Further, it is very difficult to manufacture a thin steel sheet with a thickness of not more than 2.0 mm through only the hot rolling step. In addition to the problem on the dimensional accuracy, the reduction of steel sheet temperature due to the thinning obliges the rolling of low carbon steel at a temperature below Ar.sub.3 transformation point, resulting in the conspicuous deterioration of physical properties (ductility, drawability and the like). Even if the physical properties can be ensured by the rolling below Ar.sub.3 transformation point, there is caused a new problem that the ridging is liable to occur in the steel sheet rolled at a temperature of ferrite region.
The term "ridging" used herein means an uneven defect produced on the surface of the product during the forming, which becomes fatal in this type of the steel sheet mainly used in the outermost portion of the formed article.
The ridging metallographically results from the fact that a group of crystal orientation not easily fractured even though rolling-recrystallization steps (for example, {100} orientation group) remains in the rolling direction as it is, which is generally liable to be produced at a relatively high temperature rolled state in a ferrite (.alpha.) region. Particularly, this tendency is strong when the draft at the ferrite region is high or in case of manufacturing thin steel sheets.
Lately, the formable thin steel sheets are frequency subjected to more severe forming with the complication so that they are required to have an excellent ridging resistance.
The manufacturing steps for iron and steel materials are considerably varying, which also include the case of manufacturing formable thin steel sheets.
That is, the slabbing step may be omitted by the introduction of continuouly casting process. For the purpose of improving the physical properties and saving energy, the heating temperature of slab tends to reduce from about 1,200.degree. C., which has been adopted in the prior art, to about 1,100.degree. C. or less. Also, there is gradually practised a process capable of omitting the heat treatment in the hot rolling and the rough rolling step by directly producing a steel sheet with a thickness of not more than 50 mm from molten steel.
However, all of these new manufacturing steps are disadvantageous in case of breaking a texture produced in the solidification of molten steel (casting texture). Particularly, it is very difficult to break a strong casting texture consisting mainly of {100}&lt;uvw&gt; orientation formed in the solidification. As a result, the aforementioned ridging is apt to be caused in the final thin steel sheet.
In this connection, there have been proposed some methods of manufacturing formable thin steel sheets, wherein the slab sheet is directly shaped into a thin steel sheet with a given thickness at a relatively lower temperature region of less than Ar.sub.3 transformation point and not subjected to subsequent cold rolling and recrystallization annealing steps. For example, Japanese Patent laid open No. 48-4,329 discloses that a low carbon rimmed steel is rolled into a steel sheet with a thickness of 4 mm at a temperature below Ar.sub.3 transformation point and a draft of 90% to thereby provide a yield point of 26.1 kg/mm.sup.2, a tensile strength of 37.3 kg/mm.sup.2, an elongation of 49.7% and r-value of 1.29. In Japanese Patent laid open No. 52-44,718 is disclosed a method of manufacturing low yield point steel sheet having an yield point of not more than 20 kg/mm.sup.2 by hot rolling a low carbon rimmed steel to a thickess of 2.0 mm at a final temperature of 800.degree.-860.degree. C. (below Ar.sub.3 transformation point) and coiling at a temperature of 600.degree.-730.degree. C. However, the resulting steel sheet has a conical cup value as an index for drawability of about 60.60-62.18 mm, which is equal or less in the drawability as compared with the conventionally known steel sheet having a conical cup valve of 60.58-60.61. Further, Japanese Patent laid open No. 53-22,850 discloses a method of manufacturing low carbon hot rolled steel sheet by hot rolling a low carbon rimmed steel to a thickness of 1.8-2.3 mm at a final temperature of 710.degree.-750.degree. C. and coiling at a temperature of 530.degree.-600.degree. C. However, the conical cup value of the resulting steel sheet is the same as in the aforementioned Japanese Patent laid open No. 52-44,718 and the drawability is poor. In Japanese Patent laid open No. 54-109,022 is disclosed a method of manufacturing low strength, mild steel sheets having a yield point of 14.9-18.8 kg/mm.sup.2, a tensile strength of 27.7-29.8 kg/mm.sup.2 and an elongation of 39.0-44.8% by hot rolling a low carbon aluminum killed steel to a thickness of 1.6 mm at a final temperature of 760.degree.-820.degree. C. and coiling at a temperature of 650.degree.-690.degree. C. In Japanese Patent laid open No. 59-226,149 is disclosed a method of manufacturing a thin steel sheet with r-value of 1.21 by rolling a low carbon Al killed steel comprising 0.002% of C, 0.02% of Si, 0.23% of Mn, 0.009% of P, 0.008% of S, 0.025% of Al, 0.0021% of N and 0.10% of Ti to a thickness of 1.6 mm at 500.degree.-900.degree. C. and a draft of 76% while applying a lubricant oil.